The foregoing type of fire alarm system is well-known as schematically illustrated in FIGS. 1 and 2. In such a system, as illustrated by the dashed outline, there is a responder assembly 10 and a sensor tube 11. Such sensor tube 11 may be several feet long and is placed in the compartment of an aircraft where fire or overheat conditions are detected. The sensor tube is shown in enlarged detail in FIG. 2 and includes a core element 12 which stores hydrogen gas and is spiral wrapped to allow a gas path in the event of sensor damage such as crushing or kinking. Then the wall 13 encloses the core but has sealed in pressurized helium gas.
The responder assembly 10 to which sensor tube 11 is connected basically has a gastight plenum 15 to which capillary tube 11 is connected. In a prior fire detector system sold as a Model 801-DRH by Systron Donner Corporation, the present assignee, plenum 15 was actually formed of two separate units Each unit contained either an alarm switch 14 or an integrity switch 16. The alarm switch 14 which is normally opened would close on an overheat or fire condition. This would be caused by an increase in gas pressure 11 which would force the diaphragm 17 against the contact designated 1. Similarly, if the sensor tube 11 was cut, which would release its gas pressure, the diaphragm 18 which is normally closed against the contact designated 3 would open signifying failure of the system.
The remainder of the detector includes electrical circuitry connected to terminal 1 to provide a 28-volt DC voltage, terminal 2 which provides an alarm signal which is connected to metallic diaphragms 17 and 18 whenever one switch closes and the other switch opens, and terminal 3 which is a system test. The diaphragm switches 14 and 16 controlled by a sensor tube 11 is generally disclosed in one of many Lindberg, Jr. patents, a typical one of which is 3,122,728.
In operation in general, ambient helium gas pressure in the sensor tube 11 is directly related to average temperature in, for example, an engine compartment of an airplane. Engine compartment overheat causes a proportionate rise in gas pressure. When the compartment temperature rises to the factory set alarm rating, the rising gas pressure closes the sensor alarm switch 14. When compartment cooling reduces the gas pressure the alarm switch opens and is ready to respond again. For indication of an actual fire rather than overheat conditions, hydrogen gas in the core 12 (FIG. 2) is released to close the alarm switch. Lastly when the sensor tube 11 is cut, the helium gas escapes and the integrity switch 16 opens.
To structurally implement the showing of FIG. 1, the above-mentioned detector Model 801-DRH utilized two separate side-by-side responder assemblies, each including its own separate plenum and diaphragm switch, which then were connected to a common sensor tube.
Since this detector is for aircraft applications, minimization of both weight and size is important.